Valve regulated (“sealed”) lead acid (VRLA) batteries are known. These batteries include a plurality of positive and negative plate electrodes, as in a prismatic cell, or include layers of separator and electrode tightly wound together, known as “jelly roll” cells. The electrodes within a battery are arranged so that they alternate, negative-positive-negative, etc., with separator material between adjacent plates.
The separator, typically composed of a mat of fiberglass, serves several purposes. These include to retain electrolyte and to electrically insulate one electrode from the other. In addition, the partial saturation of the separator allows for void spaces through which gaseous oxygen may be transferred from a positive electrode, where it is generated, to a negative one, where it is consumed and reincorporated into the electrolyte. This is generally referred to as the internal oxygen cycle.
In order to establish an efficient oxygen cycle, the electrolyte should be immobilized. There are two known techniques to achieve this goal. One consists of absorbing the electrolyte within a fine fiber absorbent separator; the other is solidifying/gelling it by reaction with fine particles of, typically, fumed or colloidal silica. In this latter technique, the battery requires microporous separators to keep opposing electrodes apart. Various aspects regarding the process of gelling the electrolyte cause practical difficulties.
Some of the difficulties arise because sulfuric acid electrolyte reacts with fine silica particles quite readily. To prevent premature gel formation prior to filling the battery with a mixture of electrolyte and silica particles, the mixture may either be stirred, or chilled to about 15 degrees Celsius below the ambient temperature. Further, the amount of fine silica particles needed to gel sulfuric acid depends on the acid concentration. Thus, if a dilute sulfuric acid solution is desirable, a much larger amount of silica particles will be necessary to initiate gelation. An aqueous solution of sulfuric acid, having a specific gravity of 1.280 will require 6% of its weight in fumed silica particles to form a gel. A weaker solution, having a specific gravity of about 1.050, will require about three times as much silica to form a gel.
Two processes are currently available to the battery manufacturer to produce batteries with gelled electrolyte. One process entails assemblage of a battery with its plates already electrochemically charged, followed by filling the battery with an electrolyte having a specific gravity in the range of 1.240–1.260. Approximately 6%–9% of fine silica particles are then added to the electrolyte. To prevent gel formation, the mixture must be kept continuously stirred and/or chilled 15–20 degrees Celsius below the ambient temperature. Once filled, the batteries are recharged and the electrolyte specific gravity rises to a range between 1.270 and 1.300. The second process includes the assembly of a battery with electrochemically uncharged plates. The plates are then treated with an electrolytic solution with specific gravity in the range of 1.200–1.240 to “form” the battery. Following formation, the battery is completely discharged. In this state, the sulfate content of the plates is known to be quite high. The electrolytic solution used during formation is then removed and the battery is refilled with a mix of sulfuric acid solution, with specific gravity in the range of 1.050–1.100, and 12%–18% (by weight) of fine silica particles. The battery is then recharged. Upon completion of recharging, the electrolyte/silica will be gelled. The electrolyte specific gravity will have reached a level of 1.280–1.300.
A need exists for a battery separator which overcomes these drawbacks to current technology.